1. Field of the Invention
The present invention relates to a one part moisture cure polyurethane sealant which is cured under an ordinary temperature by reaction with moisture contained in the air, and more particularly, it relates to a sealant containing urethane prepolymer.
2. Description of the Prior Art
A one part moisture cure polyurethane sealant is a useful sealant which is cured under an ordinary temperature by reaction of an isocyanate group contained in urethane prepolymer and moisture contained in the air. The cure rate and storage stability of such a sealant depend on the type and function of a cure accelerating catalyst (hereinafter simply referred to as "catalyst").
In general, catalysts accelerating curing of urethane prepolymer are classified into:
(1) naphthenate or octenate salts of heavy metals such as tin, lead, bismuth, cobalt and manganese;
(2) heavy metal organic compounds such as dibutyl tin malate and dibutyl tin dilaurate; and
(3) tertiary amine and other nitrogen-containing materials such as N-alkyl morpholine, N-alkyl aliphatic polyamine, N-alkyl piperazine and triethylenediamine.
A one part sealant in a closed container must have storage stability which is ensured at least six months under an ordinary temperature, in addition to quick curability. The tertiary amine and other nitrogen-containing compounds classified in the above item (3) are considerably problematic in storage stability as compared with the compounds classified in the items (1) and (2), and inferior in curability under low-temperature/low-humidity conditions (lower than 10.degree. C. and 50% R.H.).
For example, U.S. Pat. Nos. 4,574,793 and 4,758,648, the disclosures of which are incorporated herein by reference, employ such tertiary amine and other nitrogen-containing compounds.
In consideration of curability, workability, storage stability and sealant properties after curing of such a one part polyurethane sealant, dibutyl tin malate, dibutyl tin dilaurate and the like classified in the above item (2) are empirically employed as catalysts for commercially available one part polyurethane sealants of high performance, in the present circumstances.
U.S. Pat. No. 3,391,101, the disclosure of which is incorporated herein by reference, relating to a two part polyurethane sealant obtained by mixing urethane prepolymer and polyether polyol immediately before employment is basically different from the inventive one part polyurethane sealant, and lists a large number of chemical names relating to catalysts.
U.S. Pat. No. 4,758,648, the disclosure of which is incorporated herein by reference, describes well-known catalysts etc. for a one part polyurethane sealant, but the same mentions no catalyst synthesized according to the present invention.
When a catalyst belonging to the above item (2) is employed, the cure rate is extremely reduced under low-temperature/low-humidity conditions and extremely increased under high-temperature/high-humidity conditions. That is, too much difference is caused in the cure rate between winter and summer seasons, to hinder work control.
Although a catalyst for a one part moisture cure polyurethane sealant has an extremely important function of accelerating curing, presence of such a catalyst is rendered unpreferable once a sealing composite is cured. That is, when the cured sealing composite is exposed to extremely severe aging conditions such as high-temperature/high-humidity conditions or too strong ultraviolet rays, the catalyst once serving as a cure accelerator breaks a polyurethane linkage or a urea linkage as a harmful matter accelerating depolymerization this time. While functions of catalysts are extremely complex in general and theoretical clarification thereof is difficult, it is readily imaginable that, if a cured sealing composite is exposed to extremely severe aging conditions, the catalyst contained in the composite itself may be thermally dissociated or hydrolyzed.
Considering the result of thermal dissociation or hydrolysis caused in the catalyst contained in the cured sealing composite under extremely severe conditions, an acid substance such as a COOH radical is formed as the result of thermal dissociation or hydrolysis since every counterpart for a heavy metal is an organic acid having a COOH radical as chemical structure common to the compounds belonging to the above items (1) and (2), to exert bad influence on an adhered surface which is brought into contact with the sealant. It can be readily inferred that, when a polyurethane sealant is applied to a steel plate for an automobile, the presence of such a COOH radical caused by thermal dissociation or hydrolysis of the catalyst contained in the cured sealing composite itself exerts bad influence on rusting and embrittlement of the steel plate.
In order to develop an excellent one part sealant having good properties such as quick curability, storage stability and the like, therefore, it is necessary to synthesize a novel catalyst and blend the same with a sealing composite.